On Multipole - PowerPoint Presentation

Slide1

On

Multipole Measurements, Optics and Magnet Swapping

To Swap, or not to Swap, that is the question:
Whether 'tis Nobler in the mind to suffer
The Slings and Arrows of outrageous Fortune,
Or to take Arms against a Sea of troubles…

Apologies to W. Shakespeare.

Glen White, SLAC

July 6 2012Slide2

Overview

Summary of status of multipole measurements and inclusion in simulation modelsReminder of swapping criteria and expected effect on ATF2 opticsTracking studies for current optics with and without swapsDiscussion and recommendations for swap decision

Discussion and recommendations for removal of QD0/QF1 BPMsSlide3

Status of Multipole Measurements

Last exhaustive study by Mark and Mika in March 2011Confirmed details of measurement (co-ordinate system, direction of coil rotation, units used etc) and confirmed proper implementation in MAD description.Tracking cross-checked MAD, SAD, MADX and

Lucretia.http://www.slac.stanford.edu/~mdw/DropBox/baisha/Results of multipole determination were incorporated in v4.4 of ATF2 lattice (and carried forward to v4.5)http://code.google.com/p/atf2flightsim/source/browse/#svn%2Ftrunk%2FATF2%2FFlightSim%2FlatticeFiles%2Fsrc%2Fv4.5Slide4

Reliability of Multipole Measurements

Most QEA’s had reproducibility few 1e-4Last batch not so good, but IHEP measurements were in some way “rushed” and we perhaps have good reason to discard these…

Expected level of accuracy of tilt measurements ~0.2mrad

… courtesy of Mika and ShaSlide5

Sensitivity to Errors on Multipole Measurements (BX2.5BY1 optics and old (worse) multipole

definitions)Effect on IP beam size of absolute errors on multipoles

.Expect weak scaling with errors of order those expected.Slide6

QEA Magnet Swap Suggestion By Edu Marin

Proposed swaps based on skew-sextupole tolerances (“Swap 1”)Slide7

Effect of Magnet Swap on Tracked Beam size for optimized lattice(v4.5 BX2.5BY1 lattice, emit_x

=2nm)Very little effect for this configuration.Need to check for larger

emit_x (3-4nm)Slide8

ATF2 Tracking Models

Consider 4 optics configurationsBX10BY1Optics optimised for 40mm x

0.1mm IP beta with quads tuned to give ~35nm IP sigma_yBX10BY1(S)As above but with the proposed magnet swapsBX10BY1*BX2.5BY1 optics re-matched to BX10BY1 just using matching quads and considering linear TwissThe running configuration for May and JuneBX10BY1*(S)As above but with the proposed magnet swaps

Consider 4 beam configurationsNominal conditions (matched and 2nm x 12pm emittance)T + ETwiss

and

emittance

as measured by OTR 5/25 (3.8nm

x

25pm

emittance

)

T

Just alter

twiss

parameters to OTR measured values

E

Just change input

emittances

Tracking by

Lucretia

Found 10k macro-particles gives unstable answers for some of these configurations, changed to using 20k which is more stable, some numbers changed a little since last presentation.Slide9

Tracking Results

Config

BX10BY1BX10BY1(S)BX10BY1*BX10BY1*(S)T + E (σx /um) (σy / nm)

10.257.6 / 68.59.860.9 / 70.09.081.3 / 110.2

8.9

76.8 / 100.9

T

(

σ

x

/um)

(σ

y

/ nm)

6.9

36.3 / 37.6

6.5

39.9 / 43.4

6.3

50.0

/ 61.8

6.2

48.3 / 57.2

E

(σx /um) (σy / nm)12.354.0 / 55.412.154.5 / 57.713.070.0 / 83.013.067.0 / 76.4Nominal (σx /um) (σy / nm)8.836.0 / 36.58.835.8 / 37.19.444.3 / 49.49.543.3 / 46.6

IP

y

sizes shown “gauss fit / RMS”

Swapping magnets

For BX10BY1 no change for nominal, slightly worse for others

For BX10BY1*

slightly improved in all cases

Effect

of

Twiss

mismatch

Minimal

for BX10BY1,

significantly degrades IP beam sizes for BX10BY1*

Effect

of

emittance

dilution

Severe

in both optics cases

Optics

matching effect

BX10BY1

significantly better than BX10BY1* everywhereSlide10

Considerations For Swapping QEA Magnets

Already engaged in hardware program to improve IPBSM over summer.Swapping magnets (if taken into account with optics tuning) can only help (no downside for optics, tuning etc) in principal, if only marginally (for Goal 1)But swap procedure is not risk-free, some damage could occur during swaps, databases need to be changed, mistakes could be made etc

All other things being equal, would prefer to change 1 thing at a time (IPBSM and magnet configurations)Should only perform swap if there is a clear benefit for doing so…Slide11

Reasons for Swapping QEA Magnets

If we decide to trust in the multipole measurementsOptics re-matching and simulations show desired beam size reachable without swappingTuning simulation also shows tuning should work (see talk in 13

th ATF2 project meeting)Can we simplify re-matching if swap magnets?Maybe, what is benefit? Improved tolerance to errors greater than that considered in simulation so far? Would need more simulations to properly assess…If we believe multipoles are considerably larger than that of our current understandingIn this case swapping may have larger benefit than that considered here (for Goal 1)No clear evidence as of now that the multipole components, as installed, are any different than indicated by the measurements.

In this case, then we have no clear justification as to which swaps to make, would need to re-measure all magnetsMy suggestionThe risks in performing the swap (this summer) out-weigh any potential benefits and we should deferSlide12

Considerations for QD0/QF1 BPM Removal

It has been observed that the S-BPM feed-throughs have non-negligible magnetic fields (few 100 Gauss?)Should we split the magnets and remove the BPMs

?See previous slide on wanting to keep magnet setup unchanged if possibleRemoving BPMs not un-tolerable, have 2 other BPMs in final doublet area, but all other things being equal would prefer to keep if possibleFeed-through is ~5cm upstream from magnet coils, is this measured field expected to have an effect on the multipole components in a noticeable way?Rough impression from Cherrill Spencer (SLAC magnet engineer) is “probably not”

RecommendationBefore deciding to remove BPMs, do more study to evaluate effect of the feed-through field on the magnet multipolesMake more measurements between the feed-through and the magnet above and below the

beamline

Over the summer SLAC can use such data to try and evaluate expected effect on

multipole

components

Can also consider other remedies?

Remove feed-through in-place

Wrap feed-through in magnetic shielding material (can do with beam tests to asses effect)